adding leetcode 208, trie problem

leetcode/p208_trie.cpp

@@ -0,0 +1,97 @@
+class TrieNode {
+public:
+    // initializing one node
+    TrieNode(){
+        is_word_complete = false;
+        // each node can have at most 26 children: a to z.
+        for(int i=0;i<26;i++){
+            children.push_back(nullptr);
+        }
+    }
+    // so that we can access private member variables.
+    friend class Trie;
+private:
+    bool is_word_complete;
+    std::vector<TrieNode*> children;
+};
+
+class Trie {
+public:
+    // initializing the prefix tree
+    Trie() {
+        // which basically is initializing the root
+        root = new TrieNode;
+    }
+    
+    void insert(string word) {
+        int len = word.length();
+        TrieNode* current = root;
+        for(int i=0; i<len; i++){
+            // inserting a node at the branch of [word[i]-'a'], as the children of current
+            // if no one has inserted here yet
+            if(current->children[word[i]-'a'] == nullptr){
+                // create a node
+                TrieNode* node = new TrieNode;
+                current->children[word[i]-'a'] = node;
+                // after the insertion, update current
+                current = current->children[word[i]-'a'];
+            }else{
+                // else, it means this node is already there, just update current
+                // we make the code duplicated here, but this way it's easier to understand.
+                current = current->children[word[i]-'a'];   
+            }
+        }
+        // when we get out of the for loop, the whole word should have been inserted already, we mark this node as the end of a word.
+        current->is_word_complete = true;
+    }
+    
+    bool search(string word) {
+        int len = word.length();
+        // once again, starting from root
+        TrieNode* current = root;
+        for(int i=0; i<len; i++){
+            // if this branch we are looking for doesn't exist
+            if(current->children[word[i]-'a'] == nullptr){
+                return false;
+            }else{
+                // else update current and keep checking never level
+                current = current->children[word[i]-'a'];
+            }
+        }
+        // when we get out of the for loop, the whole word is found, but we still need to make sure this node marks the end of a previously inserted word.
+        if(current->is_word_complete == true){
+            return true;
+        }else{
+            // else, it will be like, we are searching for app, but previously we inserted an apple.
+            return false;
+        }
+    }
+    
+    bool startsWith(string prefix) {
+        int len = prefix.length();
+        // once again, starting from root
+        TrieNode* current = root;
+        for(int i=0; i<len; i++){
+            // if this branch we are looking for doesn't exist
+            if(current->children[prefix[i]-'a'] == nullptr){
+                return false;
+            }else{
+                // else update current and keep checking never level
+                current = current->children[prefix[i]-'a'];
+            }
+        }
+        // since we are only searching for a prefix, when we get here, clearly we found it.
+        return true;
+    }
+private:
+    // just like every tree, we should have a root.
+    TrieNode* root;
+};
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * Trie* obj = new Trie();
+ * obj->insert(word);
+ * bool param_2 = obj->search(word);
+ * bool param_3 = obj->startsWith(prefix);
+ */